Title:
Variation of In-Place Concrete Strength in Structures
Author(s):
F. Michael Bartlett and James G. MacGregor
Publication:
Materials Journal
Volume:
96
Issue:
2
Appears on pages(s):
261-270
Keywords:
compressive strength; concrete cores; evaluation; tests
DOI:
10.14359/454
Date:
3/1/1999
Abstract:
The variation of in-place strength in a structure is due to within-batch variation, batch-to-batch variation, systematic within-member strength variation, and systematic between-member strength variation. Batch-to- batch variation is particularly significant for cast-in-place structures, and may either inflate the within-member variation if each member is cast from many batches, or inflate the between-member variation if each member is cast from a single batch. Values of coefficients of variation that represent the overall variation of the in-place concrete strength in a structure vary from 7 percent for one member cast from one batch of concrete to 13 percent for a structure consisting of many members cast from many batches of cast-in-place concrete. Multiple regression analysis techniques are used to assess the systematic variation of the strength of concretes in laboratory specimens cast from one batch of concrete. Statistically significant systematic strength variation is detected over the height of 32 of 43 columns with average strengths from 2200 to 5200 psi. Typically, the top region was 3 to 14 percent weaker than the region in the middle, and the bottom region was 3 to 9 percent stronger than the region in the middle. Significant systematic variation of the in-place strength is also detected in 20 of 26 beams, blocks, slabs, and walls with average strengths from 2200 to 17,000 psi. Investigation of ultrasonic pulse velocity and pulloff test data from building columns and bridge girders corroborates the findings of the investigation of elements cast in the laboratory.